Conventional solar cells have made significant progress in the past few decades. These solar cells rely on a semiconductor with a single band gap to absorb sunlight as illustrated in FIG. 1(a). The absorbed photons in turn excite electrons across the band gap, creating an electron-hole pair. Typically, the built-in electric field in a pn junction separates this electron and hole, which will enter an external circuit to power an electric device. The most widespread conventional solar cells use inorganic semiconductors such as Si, CdTe, and GaAs as the light-absorbing semiconductor. Devices based on these semiconductors have reached efficiencies as high as 28%. A new semiconductor, methylammonium lead iodide (CH3NH3PbI3), has attracted significant attention recently by achieving efficiencies as high as 20%, up from 4% within only about four years. Both light absorption and charge separation typically occur within the semiconductor in Si-, CdTe-, GaAs-, and CH3NH3PbI3-based devices.
However, another class of photovoltaic (PV) cells utilize organic materials for light absorption, while a wide-gap semiconductor is used to transport photo-excited carriers away from the absorption site. The advantage of organic PVs is the fact that organic materials are generally cheaper, more flexible and more tunable than their inorganic counterparts. Nevertheless, their efficiencies are usually lower (reaching only ˜12%) due to low charge carrier mobilities associated with organic materials. Regardless of whether inorganic or organic materials are used, they share common challenges associated with conventional solar cells: how to decrease materials processing and cell fabrication costs, increase light absorption, increase the energy collected per carrier, and improve the collection efficiency of the photo-excited carriers. And then, even if all of these challenges are resolved, the highest power conversion efficiencies that these conventional devices can reach is only ˜34%.
The efficiency of current PV technologies is approaching the Shockley-Queisser limit. This remarkable increase in efficiency has been the result of advances in materials processing, improving interfaces, coming up with innovative ways to decrease electron-hole recombination, and increasing light absorption. However, the scalability of PV technologies will be limited after reaching this theoretical efficiency. At that point, the only way one can improve the viability of conventional PV utilization would be by cutting costs. This important limitation has led to a search for alternative conceptual frameworks that could yield technologies that defy this theoretical limit.
One promising direction for going beyond the Shockley-Queisser limit is to use multi-junction or tandem solar cells as illustrated in FIG. 1(b). Tandem solar cells stack a number of different solar cells with semiconductors of different band gap on top of each other. The cells with the highest band-gap semiconductors are usually on top, i.e., the side that faces the sunlight. This design ensures that the low-energy light which remains unabsorbed by the uppermost cells can be absorbed by the lower-lying cells with lower band gaps. Of course, the Shockley-Queisser limit is still relevant for individual cells in this design. However, combining all of these cells together leads to a dramatic increase in light absorption without sacrificing the energy per carrier associated with the higher energy photo-generated carriers. Tandem solar cells can ideally reach efficiencies twice as high as those of singlejunction, single-gap solar cells. In fact, currently, efficiencies higher than 40% have been achieved in triple-junction, tandem solar cells that utilize band gap engineering of III-V semiconductors. Despite the tremendous promise, tandem solar cells are still very costly to fabricate and are therefore not viable for widespread commercial use.
Another idea for going beyond the Shockley-Queisser limit is to use intermediate band semiconductors (IBSCs) in single-junction solar cells. IBSCs differ from typical semiconductors in that there exists a narrow, partially filled intermediate band within the gap that separates their valence band edge (VBE) and conduction band edge (CBE) as illustrated in FIG. 1(c). In essence, the electronic structure of these materials contains two forbidden gaps, E1g and E2g. E1g separates the VBE from the intermediate band, while E2g is the gap between the intermediate band and the CBE. The presence of the intermediate band leads to sub-band-gap light absorption across E1g and E2g in addition to the absorption that would have regardless occurred across the overall band gap Etotg.
Conceptually, an IBSC-based solar cell is equivalent to two cells (with E1g and E2g band gaps) that are connected in series, together with one cell (with band gap Etotg) that is connected to the other two in parallel. The enhanced light absorption together with the ability to sustain higher Voc increases the theoretical efficiency of a single-junction solar cell based on IBSCs to as high as ˜65%, which would correspond to an optimal Etotg of ˜2 eV, with two sub-band-gaps of 1.2 eV and 0.7 eV. Even at Etotg values higher than 3 eV, efficiencies as high as 55% can theoretically be achieved. This remarkable potential for achieving high efficiencies makes IBSCs a promising class of materials for solar energy conversion. However, only a limited number of IBSC materials currently exist and manufacture of such materials is cost prohibitive for commercial applications.